Method of passivating and/or removing contaminants on a low-k dielectric/copper surface

ABSTRACT

One aspect of the invention relates to a method of removing contaminants from a low-k film. The method involves forming a sacrificial layer over the contaminated film. The contaminants combine with the sacrificial layer and are removed by etching away the sacrificial layer. An effective material for the sacrificial layer is, for example, a silicon carbide. The method can be used to prevent the occurrence of pattern defects in chemically amplified photoresists formed over low-k films.

FIELD OF THE INVENTION

[0001] The present invention relates generally to semiconductor devicemanufacturing and more particularly to methods of forming metalinterconnects.

BACKGROUND OF THE INVENTION

[0002] Integrated circuits include many discrete semiconductor devices.A multi-level network of metal within dielectric overlies and connectsthese discrete devices. For many years, the metal used was generallyaluminum or an aluminum alloy. More recently, copper has been used inplace of aluminum because copper's higher conductivity improves circuitperformance.

[0003] Two major obstacles had to be overcome before copper could beused in integrated circuits. First, copper is difficult to etch in orderto form wiring patterns. Second, copper diffuses rapidly. Copper candiffuse into silicon where it can cause junction failure. Copper canalso diffuse through dielectric layers, degrading them and eventuallytraveling though them into device regions.

[0004] The problem of forming copper wiring patterns has beensuccessfully overcome using damascene processes. In a damascene process,openings that form an image of an interconnection pattern are patternedin a dielectric layer. Copper is deposited or grown within theseopenings. Polishing is used to coplanarize the dielectric layer and thecopper. This leaves a copper interconnection pattern inlaid within thedielectric layer. In a single damascene process, the dielectric ispatterned through. Inter-level contacts are formed with one depositionand polishing step, and wiring with another deposition and polishingstep. In a dual damascene process, the dielectric is patterned with bothtrenches and vias, whereby both inter-level contacts and wiring can beformed with a single metal deposition and polishing step. Regardless ofwhich type of process is used, multiple layers are formed to createcomplex interconnection patterns.

[0005] The problem of copper diffusion into silicon and dielectriclayers has been overcome using diffusion barriers. Layers of diffusionbarrier material are provided between copper and adjacent dielectric orsilicon. A variety of barrier materials have been reported. Conductivebarrier materials include compounds of transition metals such astantalum nitride, titanium nitride, and tungsten nitride as well asvarious transition metals themselves. Dielectric barrier materialsinclude silicon nitride, silicon oxynitride, and PSG (a phophosilicateglass).

[0006] While the forgoing solutions have been implemented and copper hasnow been used for some time in integrated circuits, there is acontinuing need to improve integrated circuit performance. For severalyears, efforts to improve integrated circuit performance have focused onreplacing conventional dielectric materials, generally silicon dioxide,with low dielectric constant (low-k) materials. These materials providea lower capacitance than silicon dioxide and consequently, increasecircuit speed by decreasing the corresponding RC delay. Low-k barriermaterials, such as SiC, are generally used with low-k dielectrics toachieve the goal of lowering overall capacitance.

[0007] Unfortunately, difficulties have arisen when patterning low-kdielectric layers. Low-k dielectric layers are patterned usinglithography. Lithography refers to processes for pattern transferbetween various media. In lithography for integrated circuitfabrication, the substrate or dielectric or other film to be patternedis coated uniformly with a radiation-sensitive film, the resist. Thefilm is selectively exposed with radiation (such as visible light,ultraviolet light, x-rays, or an electron beam) through an interveningmaster template, the mask or reticle, forming a particular pattern.Exposed areas of the coating become either more or less soluble than theunexposed areas, depending on the type of coating, in a particularsolvent developer. The more soluble areas are removed with the developerin a developing step. The less soluble areas remain on the substrateforming a patterned coating. The pattern of the coating corresponds tothe image, or negative image, of the reticle. The patterned coating isused in further processing of the substrate, dielectric or other film.

[0008] One type of photoresist is a chemically amplified deep UVphotoresist. A deep UV photoresist often is employed because resolutionin lithography systems is primarily limited by diffraction of radiationpassing through the reticle. Employing the small wavelengths of deep UVlight reduces diffraction, however, it is difficult to produce deep UVlight at high intensity. To compensate, a chemically amplifiedphotoresist is used. In a chemically amplified photoresist, theradiation generates a catalyst, typically an acid, which catalyzes asolubility-changing reaction that occurs during a post-bake operationfollowing selective exposure of the photoresist to actinic radiation.Sometimes contaminants can occur which may impact negatively the patternof the resist, which then may be transferred to an underlying materialduring subsequent patterning.

[0009] Attempts have been made to remove contaminants from low-kdielectrics using conventional techniques, such as washing withsolvents. Unfortunately, conventional techniques have not proveneffective in preventing contamination of chemically amplifiedphotoresist formed over low-k dielectrics. There remains an unsatisfiedneed for a method of dealing with contamination when low-k dielectricsare employed.

SUMMARY OF THE INVENTION

[0010] The following presents a simplified summary in order to provide abasic understanding of some aspects of the invention. This summary isnot an extensive overview of the invention. It is intended neither toidentify key or critical elements of the invention nor to delineate thescope of the invention. Rather, the primary purpose of the summary is topresent some concepts of the invention in a simplified form as a preludeto the more detailed description that is presented later.

[0011] One aspect of the invention relates to a method of removingcontaminants from a low-k film. The method involves forming asacrificial layer over the contaminated film. The contaminants combinewith the sacrificial layer and are removed by etching away thesacrificial layer. An effective material for the sacrificial layer is,for example, a silicon carbide. The method can be used to prevent theoccurrence of pattern defects in chemically amplified photoresistsformed over low-k films.

[0012] To the accomplishment of the foregoing and related ends, thefollowing description and annexed drawings set forth in detail certainillustrative aspects and implementations of the invention. These areindicative of but a few of the various ways in which the principles ofthe invention may be employed. Other aspects, advantages and novelfeatures of the invention will become apparent from the followingdetailed description of the invention when considered in conjunctionwith the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIGS. 1A-1C are schematic illustrations of a resist undergoingexposure and development;

[0014] FIGS. 2A-2C are schematic illustrations of a resist undergoingexposure and development and experiencing a pattern defect due tocontamination;

[0015]FIG. 3A is a cross section diagram illustrating a defectassociated with a dielectric layer with a barrier layer lying thereover;

[0016] FIGS. 3B-3C are plan views of SEM photographs illustrating acontaminant underlying a barrier layer and a dielectric layer,respectively;

[0017]FIG. 4A is a cross section diagram illustrating an etch back ofthe barrier layer of FIG. 3A, revealing the defect associated with theunderlying dielectric layer;

[0018]FIG. 4B is a plan view of an SEM photograph illustrating theexposed defect of FIG. 4A;

[0019]FIG. 5A is a cross section diagram illustrating the dielectriclayer of FIG. 4A after substantially complete removal of the barrierlayer of FIG. 4A, wherein removal of the barrier layer results inremoval of the contaminant previously associated with the dielectriclayer;

[0020]FIG. 5B is a plan view of an SEM photograph illustrating thedielectric layer of FIG. 5A after removal of the barrier layer and thecontaminant;

[0021]FIG. 6A is a cross section diagram illustrating a seconddielectric layer overlying a barrier layer and the initial dielectriclayer after removal of the sacrificial barrier layer;

[0022]FIG. 6B is a plan view of an SEM photograph illustrating a portionof a second dielectric layer of FIG. 6A in accordance with the presentinvention, wherein contaminants associated with underlying low-kdielectric material has been eliminated;

[0023]FIG. 7 is fragmentary a cross section diagram illustrating apatterned photoresist layer free of pattern defects in accordance withthe present invention; and

[0024]FIG. 8 is a flow chart of a process according to another aspect ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention will now be described with reference to theattached drawings, wherein like reference numerals are used to refer tolike elements throughout. The present invention relates to a method ofeliminating or mitigating the negative effects of contaminants (e.g.,associated with a dielectric layer) on a resist such as a DUV resistduring semiconductor processing. According to one exemplary aspect ofthe invention, a sacrificial layer is formed over a dielectric film thatmay have one or more contaminants associated therewith. The potentialcontaminants are then removed by the removal of the sacrificial layer.Subsequently, one or more layers may then be formed over the dielectricand any subsequent patterned resist formed thereover then avoids patterndefects associated with the contaminants.

[0026] Historically, contaminants were not a significant problem withdense dielectric materials such as silicon dioxide, and with highquality diffusion barriers that were patterned with DUV resist or whenthe pattern was made with non-chemically amplified resists. However, itwas noted by the inventors of the present invention that when usinglow-k dielectric materials, DUV resist in some cases would not take apattern. That is, the desired pattern transfer from the photomask to theresist did not occur with sufficient accuracy (e.g., pattern defects).Although the exact cause of the problem is not known, it is believedthat amino by-products from previous processing steps may be causing asource of contamination.

[0027] Referring generally to FIGS. 1A-1C, a light source is shownthrough a patterned opaque plate intended to illuminate predeterminedregions in the resist that will be removed during subsequentdevelopment. When the photons impinge on regions of the resist, an acidis generated in local regions where the light is absorbed. Afterexposure to the DUV energy, the wafer is subjected to a post exposurebake (e.g., moved to a hot plate), where the acid induces subsequentchemical transformations including the changing of the solubility of theresist. Following the post exposure bake (PEB), a developer solution isplaced on the wafer to remove the resist with the higher solubility.

[0028] In particular, in FIG. 1A, a photoresist 10 is illustrated as acomposition of polymer chains 12 with protection groups 14 and photonacid generators 16. Then in FIG. 1B, the transposing of a pattern intothe resist is accomplished when acid is generated from DUV light 18through a photomask 20. A reaction is then brought between the acid andthe protection groups 14 with the PEB in FIG. 1C. This reaction convertsan un-soluble matrix into one that is soluble in common base developersand thus leads to a pattern created by dissolution of the resist 10 inthe light exposed regions (for a positive type resist, for example). Incontrast, in FIGS. 2A-2C, an exemplary illustration is provided to showhow it is believed that a base contaminate in the photoresistneutralizes the photon-generated acid before reactions can occur, whichprevents proper resist patterning and resulting in a pattern defect.

[0029] The move to low-k materials as the inter-metal dielectric hasrequired the use, in some instances, of low quality diffusion barriersin order to satisfy the overall capacitance (k-value) of the film stack.This has inadvertently affected how well contaminants are contained neartheir original location beneath the diffusion barrier film. In additionto the low quality diffusion barriers, the intrinsic porosity of low-kmaterial compounds the above problem by acting as a reservoir forcontaminants. The low-k material is expected to act as both a conduitand a storage center for contaminants generated in prior or futureprocessing steps. Considering the potential for the low-k material totrap and act as a source of contaminants, coupled with the lower qualityof some diffusion barriers, pattern defects discussed above are expectedto form in overlying photoresists due to diffusion of such contaminantsin underlying materials.

[0030] Turning to FIG. 3A, a typical integration using a low-k materialwith copper starts with a patterned low-k dielectric 30 that has beenpatterned, etched and filled with metal 32 such as copper, and thenchemically mechanically polished (CMP) to form metal lines. During theCMP process, it is speculated by the inventors of the present inventionthat CMP slurry and/or residue is left on top of the low-k material andlikely is absorbed into the surface thereof. After CMP cleaning, apassivation or barrier layer 34 is deposited over the copper 32 andlow-k film 30. In some instances, the barrier deposition is followed bythe deposition of another low-k dielectric (not shown) that will besubsequently patterned. If the photoresist used for such patterningfails to take the photon generated pattern due to the presence of adefect 36, the etch process will also fail local to the defect locationand thus the desired shape will not be generated.

[0031] It is believed by the inventors of the present invention thatsomething is either hindering the acid in the resist from beinggenerated or neutralized once it has been generated. Since some CMPprocesses use a solution with a high pH (base solution) to clean thesurface, it is possible that some of the base may be absorbed by thelow-k dielectric 30 or trapped in remaining residue and later released.Once at the surface, the base may neutralize the acid and prevent thephotoresist from taking the pattern. Another potential source identifiedby the inventors is the NH₃ pretreatment prior to the barrierdeposition.

[0032] In appreciating the problem above and the various potentialcauses of such pattern defects within the resist, the inventors of thepresent invention hypothesized that by depositing a sacrificial layer(e.g., a reactive film) over the low-k dielectric material (e.g., thecontamination source), a reaction may occur with the defect. Then, byremoving the sacrificial layer (e.g., with an etch back process) thedefect is also removed.

[0033] To test the above hypothesis, the inventors of the presentinvention prepared a test wafer. Upon depositing the low-k dielectricfilm 30 over the substrate (e.g., as the second inter-metal dielectricoverlying active circuitry), a defect 36 was found therein or otherwiseassociated therewith, as illustrated in FIG. 3A. A sacrificial film, inthis example a SiC film 34 was formed over the low-k layer 30, and thedefect 36 was again observed, as illustrated in FIG. 3B. FIG. 3Cillustrates an affect of an untreated defect on the photoresist (e.g.,lines and spaces are not formed in the photoresist in the vicinity of anunderlying defect). A partial etch back process was then performed onthe barrier layer 34 to expose a portion of the defect, as illustratedin FIGS. 4A and 4B, and finally the entire sacrificial layer 34 36 wasremoved by the remaining portion of the etch back process, and thedefect was eliminated, as illustrated in FIGS. 5A and 5B. After removalof the defect, a new barrier layer 40 is formed thereover, followed byanother low-k dielectric film 42 and a patterned photoresist 44, asillustrated in FIG. 6A. Upon evaluating the photoresist, the defectlocation is eliminated, and a potential pattern defect in thephotoresist due to the contaminant is eliminated, as illustrated in FIG.6B.

[0034] In light of the inventors' appreciation of the problem and abovetests in light of the inventors' hypothesis, FIG. 7 illustrates anexemplary device 100 formed according to one aspect of the presentinvention. Device 100 includes substrate 101, underlying dielectriclayer 103, copperfeatures 105, barrier layer 107, overlying dielectriclayer 109, and patterned resist 111. Contaminants have been removed fromthe underlying dielectric layer 103 by a process according to thepresent invention. Without such a process, contaminants could havediffused from the underlying dielectric layer 103, through the barrierlayer 107, and through the overlying dielectric layer 109, into theresist layer 111 wherein the contaminants could have affected theformation of the resist pattern.

[0035]FIG. 8 is a flow chart of a process 200 for providing a patternedlow-k dielectric film or other layer over an underlying low-k dielectricfilm according to one aspect of the present invention. Acts 201 and 203are forming a layer of sacrificial material over the underlying low-kdielectric film and then removing that sacrificial material. These actsremove contaminants from the underlying low-k dielectric film. Theprocess 200 continues with forming a barrier/etch stop layer, act 205,forming the overlying low-k dielectric film, act 207, forming a resistcoating, act 209, patterning the resist coating, act 211, and etchingthe overlying low-k dielectric film using the patterned resist coatingas a mask, act 213.

[0036] The underlying low-k dielectric film is generally provided over asemiconductor substrate. A semiconductor includes a semiconductor, whichis typically silicon. Other examples of semiconductors include GaAs andInP. In addition to a semiconductor, the substrate may include variouselements therein and/or layers thereon. These can include metal layers,barrier layers, dielectric layers, device structures, active elementsand passive elements including silicon gates, word lines, sourceregions, drain regions, bit lines, bases emitters, collectors,conductive lines, conductive vias, etc.

[0037] A low-k dielectric is a dielectric material having a dielectricconstant significantly lower than that of silicon dioxide. Examples oflow-k dielectrics include porous glasses and polyimide nanofoams. Porousglasses include organosilicate glasses (OSGs). The low-k dielectric canbe organic or inorganic. Examples or organic low-k dielectric includebenzocyclobutene, parylene, polyarylene ethers, and fluorocarbons.Examples of inorganic low-k dielectrics include porous silica,fluorinated amorphous carbon, methyl silsesquioxane, hydrogensilsesquioxane, and fluorinated silicon dioxide. Specific examples oflow-k dielectric materials include Applied Material's Black Diamond®,Novellus' Coral®, Allied Signal's Nanoglass® and FLARE®, JSR's LKD5104®,Texas Instrument's Xerogel®, and Dow Chemical's BCB® and SiLK®. Thelow-k dielectric can be applied by any suitable means, including forexample, spin coating or CVD.

[0038] The invention is suitable for removing a variety of contaminants.In one embodiment, the contaminant is a base. In a more specificembodiment, the contaminant is an amine, such as ammonia.

[0039] Act 201 is forming a sacrificial layer over the underlying low-kdielectric layer. The sacrificial layer is of a suitable type wherebythe contaminant diffuses from the underlying low-k dielectric layer intothe sacrificial layer. Preferably, the sacrificial layer reacts with thecontaminant. Preferably, the sacrificial layer can be etched withselectivity against the low-k dielectric. In one embodiment, thesacrificial layer includes a barrier material. Examples of barriermaterials include silicon carbide, silicon nitride, tantalum, tantalumnitride, titanium nitride, tungsten nitride, silicon oxynitride, and PSG(a phophosilicate glass). A barrier material for a sacrificial layerdoes not have to be the same as the barrier material used to formbarriers in the finished device. However, one advantage of using thebarrier material of the finished device to form the sacrificial layer isthat contaminants that diffuse into the sacrificial material willinclude all the contaminants that could diffuse through the subsequentlyformed barrier layer and thereby potentially affect an overlyingphotoresist.

[0040] The sacrificial layer can be of any suitable thickness. In oneembodiment, the sacrificial layer is from about 0.01 μm to about 100 μmthick. In another embodiment, the sacrificial layer is from about 0.2 μmto about 10 μm thick. In a further embodiment, the sacrificial layer isfrom about 0.3 μm to about 1 μm thick.

[0041] The sacrificial layer may be formed by any suitable process,including spin coating or CVD, for example. Mild heating may facilitatecontaminant removal. Such heating may take place during the process offorming the sacrificial layer. Optionally, however, heat can be providedto facilitate diffusion into and/or reaction of contaminants with thesacrificial film. In one embodiment, the substrate, along with thesacrificial film, is heated to at least about 100° C. In anotherembodiment, it is heated to at least about 150° C. In a furtherembodiment, it is heated to at least about 200° C. The temperature ismaintained for a short time. In one embodiment, it is maintained for atleast about 10 minutes. In another embodiment, it is maintained for atleast about 20 minutes. In a further embodiment, it is maintained for atleast about 30 minutes.

[0042] The sacrificial layer formed with act 201 is removed by act 203.Acts 201 and 203 together provide a process for cleaning the underlyinglow-k dielectric. Generally, the sacrificial layer is removed byetching. Etching can be wet or dry, but dry etching is preferred. Forexample, where the sacrificial layer is SiC, it can be dry etched usinga combination of O₂, CF₄, CH₃F, N₂ and Ar. A small portion of theunderlying low-k dielectric, typically less that about 1000 Å, issometimes removed together with the sacrificial layer.

[0043] The contaminants removed from the underlying low-k dielectric mayresult from chemical mechanical polishing. In addition to reactivecontaminants, chemical mechanical polishing may leave behind smallceramic particle. Preferable, the process of removing sacrificial layeris also effective in removing any small ceramic particles left over fromchemical mechanical polishing. This can be accomplished with a dry etchchemistry including, for example, a fluorine compound and/or Argon.

[0044] After removing the sacrificial layer, an overlying low-kdielectric layer, or a layer of another material, can be formed andpatterned. Before forming the overlying low-k dielectric layer, abarrier/etch stop layer is deposited with act 205. Generally, thisbarrier/etch stop layer has a relatively low-k value. The overlyinglow-k dielectric layer is formed with act 207.

[0045] Act 209 is forming the resist coating over the low-k dielectriclayer. Any suitable resist can be used. However, the invention isparticularly useful when the resist is chemically amplified. Preferably,the resist contains a photo-acid, such as a compound that forms an acidon exposure to deep-UV radiation. The resist coating is applied by anysuitable means, including, for example, spin coating.

[0046] Optionally, another barrier layer is provided between the resistand the overlying low-k dielectric. Such a barrier layer can provideadditional protection against contamination of the resist.

[0047] Act 211 is selectively exposing the resist to actinic radiationand developing the resist. Exposure takes place through an interveningmask or reticle. Developing the resist generally includes apost-exposure bake followed by removal of the more soluble portions ofthe resist using a suitable solvent developer.

[0048] Act 213 is etching the overlying low-k dielectric, whereby themask pattern is transferred into the overlying low-k dielectric. Forexample, where the low-k dielectric is OSG, it can be dry etch using Ar,N₂, and C₄F₈. The resist is subsequently stripped, another barrier layeris provided, and the gaps in the overlying dielectric layer are filledwith copper.

[0049] Between acts 201 and 203, providing and removing the sacrificiallayer, additional procedures can be carried out to facilitate theremoval of contaminants from the underlying low-k film. In oneembodiment, a second sacrificial layer can be provided and then removed.The second sacrificial layer can be, for example, a low-k dielectriclayer, which will generally be effective in absorbing the types ofcontaminants found in the underlying low-k dielectric.

[0050] In another embodiment, a solvent for the contaminant is appliedover the sacrificial layer. For example, a weak acid solution can beused as a solvent for amine contaminants. The sacrificial layer mayprevent the solvent from contacting the underlying low-k dielectriclayer. In addition, it should be understood that the present inventionis applicable to standard semiconductor processing as well as re-worktype processing, and both applications, as well as others, arecontemplated as falling within the scope of the present invention.

[0051] Although the invention has been illustrated and described withrespect to one or more implementations, equivalent alterations andmodifications will occur to others skilled in the art upon the readingand understanding of this specification and the annexed drawings. Inparticular regard to the various functions performed by the abovedescribed components (assemblies, devices, circuits, systems, etc.), theterms (including a reference to a “means”) used to describe suchcomponents are intended to correspond, unless otherwise indicated, toany component which performs the specified function of the describedcomponent (e.g., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure which performs thefunction in the herein illustrated exemplary implementations of theinvention. In addition, while a particular feature of the invention mayhave been disclosed with respect to only one of several implementations,such feature may be combined with one or more other features of theother implementations as may be desired and advantageous for any givenor particular application. Furthermore, to the extent that the terms“includes”, “having”, “has”, “with”, or variants thereof are used ineither the detailed description and the claims, such terms are intendedto be inclusive in a manner similar to the term “comprising.”

What is claimed is:
 1. A method of removing a contaminant from a low-kdielectric, comprising: forming a sacrificial layer over the low-kdielectric; and etching to remove the sacrificial layer.
 2. The methodof claim 1, wherein the sacrificial layer reacts with the contaminant.3. The method of claim 1, wherein the sacrificial layer comprises abarrier material.
 4. The method of claim 3, wherein the barrier materialis silicon carbide.
 5. The method of claim 1, wherein the sacrificiallayer comprises a material that reacts with ammonia.
 6. The method ofclaim 1, further comprising, after forming the sacrificial layer andbefore etching the sacrificial layer: forming a layer of a secondmaterial that absorbs the contaminant; and etching to remove the secondmaterial.
 7. The method of claim 6, wherein the second material is alow-k dielectric.
 8. The method of claim 1, further comprisingcontacting the substrate with a solvent after forming the sacrificiallayer and before etching the sacrificial layer.
 9. The method of claim8, wherein the solvent comprises an acidic solution.
 10. The method ofclaim 1, wherein etching to remove the sacrificial layer comprises dryetching with a chemistry that removes small ceramic particles.
 11. Amethod of patterning a low-k dielectric over a substrate, comprising:forming a sacrificial layer over the substrate; etching to remove thesacrificial layer; forming a low-k dielectric layer over the substrate;coating the low-k dielectric layer with a chemically amplifiedphotoresist; selectively exposing the photoresist to actinic radiation;developing the photoresist; and etching the low-k dielectric layer usingthe photoresist as a mask.
 12. The method of claim 11, wherein theresist comprises a photo-acid.
 13. The method of claim 11, furthercomprising forming a barrier layer over the substrate prior to formingthe low-k dielectric layer.
 14. The method of claim 13, furthercomprising forming a second barrier layer over the low-k dielectriclayer prior to coating with the resist.
 15. The method of claim 11,wherein etching to remove the sacrificial layer comprises dry etchingwith a chemistry that removes small ceramic particles.
 16. The method ofclaim 11, wherein the substrate comprises a second low-k dielectriclayer.
 17. The method of claim 11 wherein the sacrificial layer reactswith ammonia.
 18. The method of claim 11 wherein the sacrificial layercomprises SiC.